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N. Microorganisms, Prokaryotes, and Viruses CK-12 FOUNDATION N. Microorganisms, Prokaryotes, and Viruses Say Thanks to the Authors Click http://www.ck12.org/saythanks (No sign in required) Brainard SHUMOCK To access a customizable version of this book, as well as other interactive content, visit www.ck12.org CK-12 Foundation is a non-profit organization with a mission to reduce the cost of textbook mate- rials for the K-12 market both in the U.S. and worldwide. Using an open-content, web-based collaborative model termed the FlexBook®, CK-12 intends to pioneer the generation and distribution of high-quality educational content that will serve both as core text as well as provide an adaptive environment for learning, powered through the FlexBook Platform®. Copyright © 2011 CK-12 Foundation, www.ck12.org The names “CK-12” and “CK12” and associated logos and the terms “FlexBook®”, and “FlexBook Platform®”, (collectively “CK-12 Marks”) are trademarks and service marks of CK-12 Foundation and are protected by federal, state and international laws. Any form of reproduction of this book in any format or medium, in whole or in sections must include the referral attribution link http://www.ck12.org/saythanks (placed in a visible location) in addition to the following terms. Except as otherwise noted, all CK-12 Content (including CK-12 Curriculum Material) is made available to Users in accordance with the Creative Commons Attribution/Non-Commercial/Share Alike 3.0 Un- ported (CC-by-NC-SA) License (http://creativecommons.org/licenses/by-nc-sa/3.0/), as amended and updated by Creative Commons from time to time (the “CC License”), which is incorporated herein by this reference. Complete terms can be found at http://www.ck12.org/terms. Printed: August 22, 2011 Authors Jean Brainard, ELISABETH SHUMOCK Contributors Michelle Rogers-Estable, ELISABETH SHUMOCK i www.ck12.org Contents 1 N. Microorganisms: Prokaryotes and Viruses 1 1.1 Prokaryotes ............................................. 2 1.2 Viruses ............................................... 15 www.ck12.org ii Chapter 1 N. Microorganisms: Prokaryotes and Viruses Can you guess what organisms are pictured here? Are they fat green worms on a red leaf? Here’s a clue: There are more organisms like these than any other on Earth. Here’s another clue: Each organisms consists of a single cell without a nucleus. The organisms are bacteria called Salmonella. If the word Salmonella rings a bell, that’s probably because Salmonella causes human diseases such as food poisoning. Many other types of bacteria also cause human diseases. But not all bacteria are harmful to people. In fact, we could not survive without many of the trillions of bacteria that live in or on the human body. You will learn why when you read this chapter. 1 www.ck12.org 1.1 Prokaryotes Lesson objectives • Outline the classification and evolution of prokaryotes. • Describe the structure of prokaryotes. • Identify different types of metabolism found in prokaryotes. • Describe the range of prokaryote habitats. • Explain how prokaryotes reproduce. • Identify important relationships between bacteria and humans. Vocabulary • antibiotic drug • antibiotic resistance • Archaea • Bacteria • biofilm • cyanobacteria • endospore • extremophile • flagella • genetic transfer • Gram-negative bacteria • Gram-positive bacteria • plasmid • vector Introduction No doubt you’ve had a sore throat before, and you’ve probably eaten cheese or yogurt. If so, then you’ve encountered the fascinating world of prokaryotes. Prokaryotes are single-celled organisms that lack a nucleus. They also lack other membrane-bound organelles. Prokaryotes are tiny and sometimes bothersome, but they are the most numerous organisms on Earth. Without them, the world would be a very different place. An overview of bacteria can be seen at http://www.youtube.com/user/khanacademy#p/c/7A9646BC5110CF64/16/TDoGrbpJJ14. Figure 1.1: (Watch Youtube Video) http://www.ck12.org/flexbook/embed/view/106 www.ck12.org 2 Evolution and Classification of Prokaryotes Prokaryotes are currently placed in two domains. A domain is the highest taxon, just above the kingdom. The prokaryote domains are Bacteria and Archaea (see Figure 1.2). The third domain is Eukarya. It includes all eukaryotes. Unlike prokaryotes, eukaryotes have a nucleus in their cells. Figure 1.2: The Three Domains of Life. All living things are grouped in three domains. The domains Bacteria and Archaea consist of prokaryotes. The Eukarya domain consists of eukaryotes. Prokaryote Evolution It’s not clear how the three domains are related. Archaea were once thought to be offshoots of Bacteria that were adapted to extreme environments. For their part, Bacteria were considered to be ancestors of Eukarya. Scientists now know that Archaea share several traits with Eukarya that Bacteria do not share (see Table 1.1). How can this be explained? One hypothesis is that Eukarya arose when an Archaean cell fused with a Bacterial cell. The two cells became the nucleus and cytoplasm of a new Eukaryan cell. How well does this hypothesis fit the evidence in Table 1.1? Table 1.1: Comparison of Bacteria, Archaea, and Eukarya Characteristic Bacteria Archaea Eukarya Flagella Unique to Bacteria Unique to Archaea Unique to Eukarya Cell Membrane Unique to Bacteria Like Bacteria and Eu- Unique to Eukarya karya Protein Synthesis Unique to Bacteria Like Eukarya Like Archaea Introns Absent in most Present Present Peptidoglycan (in cell Present Absent in most Absent wall) 3 www.ck12.org Domain Bacteria Bacteria are the most diverse and abundant group of organisms on Earth. They live in almost all environ- ments. They are found in the ocean, the soil, and the intestines of animals. They are even found in rocks deep below Earth’s surface. Any surface that has not been sterilized is likely to be covered with bacteria. The total number of bacteria in the world is amazing. It’s estimated to be 5 × 1030, or five million trillion trillion. You have more bacteria in and on your body than you have body cells! Bacteria called cyanobacteria are very important. They are bluish green in color (see Figure 1.3) because they contain chlorophyll. They make food through photosynthesis and release oxygen into the air. These bacteria were probably responsible for adding oxygen to the air on early Earth. This changed the planet’s atmosphere. It also changed the direction of evolution. Ancient cyanobacteria also may have evolved into the chloroplasts of plant cells. Figure 1.3: Cyanobacteria Bloom. The green streaks in this lake consist of trillions of cyanobacteria. Excessive nutrients in the water led to overgrowth of the bacteria. Thousands of species of bacteria have been discovered, and many more are thought to exist. The known species can be classified on the basis of various traits. One classification is based on differences in their cell walls and outer membranes. It groups bacteria into Gram-positive and Gram-negative bacteria, as described in Figure 1.4. Domain Archaea Scientists still know relatively little about Archaea. This is partly because they are hard to grow in the lab. Many live inside the bodies of animals, including humans. However, none are known for certain to cause disease. Archaea were first discovered in extreme environments. For example, some were found in hot springs. Others were found around deep sea vents. Such Archaea are called extremophiles, or ‘‘lovers of extremes.” Figure 1.5 describes three different types of Archaean extremophiles. The places where some of them live are thought to be similar to the environment on ancient Earth. This suggests that they may have evolved very early in Earth’s history. Archaea are now known to live just about everywhere on Earth. They are particularly numerous in the ocean. Archaea in plankton may be one of the most abundant types of organisms on the planet. Archaea www.ck12.org 4 Figure 1.4: Classification of Bacteria. Different types of bacteria stain a different color when stained with Gram stain. This makes them easy to identify. are also thought to play important roles in the carbon and nitrogen cycles. For these reasons, Archaea are now recognized as a major aspect of life on Earth. Prokaryote Structure Most prokaryotic cells are much smaller than eukaryotic cells. Although they are tiny, prokaryotic cells can be distinguished by their shapes. The most common shapes are helices, spheres, and rods (see Figure 1.6). Plasma Membrane and Cell Wall Like other cells, prokaryotic cells have a plasma membrane (see Figure 1.7). It controls what enters and leaves the cell. It is also the site of many metabolic reactions. For example, cellular respiration and photosynthesis take place in the plasma membrane. Most prokaryotes also have a cell wall. It lies just outside the plasma membrane. It gives strength and rigidity to the cell. Bacteria and Archaea differ in the makeup of their cell wall. The cell wall of Bacteria contains peptidoglycan (composed of sugars and amino acids). The cell wall of most Archaea lacks peptidoglycan. Cytoplasm and Cell Structures Inside the plasma membrane of prokaryotic cells is the cytoplasm. It contains several structures, including ribosomes, a cytoskeleton, and genetic material. Ribosomes are sites where proteins are made. The cytoskeleton helps the cell keeps its shape. The genetic material is usually a single loop of DNA. There 5 www.ck12.org Figure 1.5: Extremophile Archaea. Many Archaea are specialized to live in extreme environments. Just three types are described here. Figure 1.6: Prokaryotic Cell Shapes. The three most common prokaryotic cell shapes are shown here. www.ck12.org 6 may also be small, circular pieces of DNA, called plasmids. (see Figure 1.8). The cytoplasm may contain microcompartments as well. These are tiny structures enclosed by proteins. They contain enzymes and are involved in metabolic processes. Extracellular Structures Many prokaryotes have an extra layer, called a capsule, outside the cell wall.
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